Mac OS X Tip of the Day: Move your Home Directory
There's a very powerful tool, called NetInfo Manager, tucked away in the Utilities folder. NetInfo is the system database that manages key system settings (like users and groups properties).
Anyways, with NetInfo Manager, you can navigate through the users directory to select your account. There, you'll see several properties including 'home'. Changing this value will allow you put your home directory on a different hard drive (or partition).